Antenna system

ABSTRACT

A microwave antenna assembly capable of directing microwave energy in both the elevational and azimuthal directions and especially adapted to satellite communications wherein the microwave energy source and its attendant electronic circuitry are mounted in a stationary fashion. A rotatable microwave feed portion is interposed between the microwave energy source and the antenna which consists of a subreflector and a main reflector. The feed portion is independently rotatable in first and second transverse directions to obtain the desired elevational and azimuthal sweeps and consists of reflectors which guide the microwave energy from the source to the focus of the subreflector so that the microwave energy impinging upon the subreflector has a radiation pattern substantially identical to the radiation pattern of electromagnetic waves emitted from the source. This is accomplished through the employment of a pair of offset curved reflectors which have an inverted image relationship therebetween.

United States Patent n91 Soma et al.

[451 Oct. 29, 1974 ANTENNA SYSTEM [75] Inventors: Shoji Soma; Ikuro Sato, both of Tokyo, Japan 22 Filed: Mar. 1, 1973 211 App]. No.: 336,957

[30] Foreign Application Priority Data Mar. 8, 1972 Japan 47-2316] [52] US. Cl. 343/76], 343/78] [51] Int. Cl. H0111 3/12, HOlq l9/l4 [58] Field of Search 343/76], 781, 837, 839

[56] References Cited OTHER PUBLICATIONS Kitsuregawa et al., Design of the Beam-Waveguide Primary Radiators of the Cassegrain Antennas for Satellite Communications, 9/ 16/70, I970 G-AP International Symposium, Columbus Ohio, USA, pp. 400, 401 and 404 relied upon.

Primary Examiner-Archie R. Borchelt Assistant ExaminerT. N. Grigsby Attorney, Agent, or Firm-Ostrolenk, Faber, Gerb & Soffen [57] ABSTRACT A microwave antenna assembly capable of directing microwave energy in both the elevational and azimuthal directions and especially adapted to satellite communications wherein the microwave energy source and its attendant electronic circuitry are mounted in a stationary fashion. A rotatable microwave feed portion is interposed between the micro wave energy source and the antenna which consists of a subreflector and a main reflector. The feed portion is independently rotatable in first and second transverse directions to obtain the desired elevational and azimuthal sweeps and consists of reflectors which guide the microwave energy from the source to the focus of the subreflector so that the microwave energy impinging upon the subreflector has a radiation pattern substantially identical to the radiation pattern of electromagnetic waves emitted from the sourcev This is accomplished through the employment of a pair of offset curved reflectors which have an inverted image relationship therebetween 2 Claims, 10 Drawing Figures ANTENNA SYSTEM The present invention relates to an antenna system for communication use and, more particularly, to a microwave antenna system for satellite communication systemsv An antenna system for an earth station of the satellite communication system has heretofore been composed of a dual reflector antenna such as the Cassegrain antenna and the Gregorian antenna. The antenna of this type consists of a main reflector, a subreflector, a primary feed for supplying high-frequency power to the antenna, a tracking receiver and a communication equipment. In order to minimize the line loss of the primary feed and noise. the low noise receiver for reception and the power amplifier for transmission must be disposed as closely to the antenna as possible. In the control of the antenna elevation and azimuth angles, the antenna should therefore be driven integrally with the primary feed and the electronic device. In a lownoise microwave antenna, this leads to a greater overall dimension and inconveniences in the installation and maintenance work.

An object of the present invention is therefore to provide an antenna system capable of high efficiency and low noise suited forfacilitating the function of driving the antenna to attain desired elevation and azimuth angles.

Another object of the present invention is to provide an antenna system adapted to install an electronic device separately from the movable part of the antenna system.

Still another object ofthe present invention is to provide an antenna system whose radiation pattern and efficiency are not affected by the driving of the antenna to achieve various elevation and the azimuth angles.

In accordance with the present invention, there is provided a microwave antenna system having a microwave source for radiating microwave energy in a predetermined direction with a preset disperse angle, a dual reflector antenna having a main reflector and a subreflector. and microwave path for guiding the microwave energy to the antenna so that the microwave energy may be transmitted through the antenna most efficiently, wherein the microwave path comprises; a first plane reflector for deflecting the microwave energy by 90; a first curved reflector for deflecting the microwave energy by a predetermined angle; a second curved reflector for deflecting the microwave energy reflected from the first curved reflector; a second plane reflector for directing the reflected microwave energy supplied from the first plane reflector to the subreflector of the dual reflector antenna; means for supporting the first plane reflector and first and second curved reflectors to permit a rotation of these reflectors around an axis coincident with the axis of the microwave energy emanating from the microwave source; means for supporting the dual reflector antenna and the second plane reflector to permit a rotation of the antenna and the second plane reflector around the axis lying at right angle to the axis of the first-mentioned rotation; whereby the firstand second-mentioned rotations provide the control of the azimuthal elevational direction ofthe dual reflector antenna without causing any transformation of the transmission mode of the microwave energy.

The present invention will now be described in detail with reference to the accompanying drawings, in which:

FIGS. 10 and lb show in longitudinal section the construction of a prior-art dual reflector antenna;

FIG. 2 shows a longitudinal section of the construction ofa prior-art antenna system mounted on a pedestal;

FIG. 3 shows a schematic longitudinal section of an embodiment of the present invention;

FIGS. 40 and 4b are cross-sectional views showing the ray-trace of microwaves and the arrangement of reflecting surfaces in case where the reflecting surfaces of a curved reflector are located in the relation of image by inversion;

FIGS. 50 and 5b are diagrams illustrating the principle, showing mode changes of reflected microwaves due to the curved reflector;

FIG. 6 shows in longitudinal section the construction of another embodiment of the present invention; and

FIG. 7 is a sectional view showing the transmission paths of microwaves and the positions of reflecting sur faces in the case where curved reflectors are located in the relation of rotational symmetry with each other.

Referring to FIGS. la and lb, which show longitudinal sections of the conventional dual reflector antenna. the Cassegrain antenna of FIG. la has a paraboloid of revolution as a main reflector 3, while a hyperboloid of revolution is employed as a subreflector 2. On the other hand, the Gregorian antenna of FIG. lb has the main reflector 3 of a paraboloid of revolution, while the subreflector 2 is an ellipsoid of revolution. When the throat of an electromagnet primary horn l is brought into coincidence with one of the foci ofthe subreflector 2 to irradiate the subreflector surface 2 by spherical waves radiated from the primary horn with small flare angle, the waves reflected by the subreflector surface are transformed to spherical waves dispersing at an acute angle as if it were radiated from the other of the foci of the subreflector surface. and are projected onto the main reflector surface 3. Accordingly, if the design is arranged so that the other focus of the subreflector surface 2 corresponds to the focus of the paraboloid of revolution constituting the main reflector surface. the spherical waves are changed into plane waves of very sharp directivity by the main reflector surface 3. The term dual reflector antenna" signifies an antenna which comprises the main reflector and the subreflector in combination. Since the dual reflector antenna does not require matching etc.. it does not pick up thermal noise from the ground due to the spill-over radiation resulting from the matching etc., and hence, it is often used for satellite communications,

In addition to the Cassegrain and Gregorian antenhas the dual reflector antenna covers one in which the distribution of the amplitudes and phases of the electromagnetic field on its aperture plane is made uniform, to make the mainand sub-reflector surfaces highly efficient (For the details of this antenna, reference is made to a paper by V. Galindo published in the IEEE Trans. Vol. AP 12, No. 4, page 403, July 1964).

In FIG. 2 showing the construction of a priorart antenna system. the principal antenna portions are. the primary horn l, the subreflector 2 and the main reflector 3, as in the case of FIG. la Reference numeral 4 indicates a feed portion for detecting the tracking error signal and for duplexing, transmitting and receiving signals', 5, a transmitter/receiver; and 6, an antenna ground base.

So far as only the electrical performances are concerned, this type of antenna system is advantageous in that the principal antenna portions and the primary horn can be coupled so as to obtain an axially symmetrical radiation pattern. Generation of axially symmetrical modes can easily be obtained by forming the electromagnetic horn i into a corrugated conical horn, so that both the main beam of the antenna and a tracking null pattern can be made axially symmetrical. More specifically, a radiation pattern from the primary feed portion hardly contains side lobes, so that noise from the ground is scarcely picked up and both the beam axes and the null axis (the center axis of the tracking pattern) are in good coincidence with each other. The above-mentioned antenna system has therefore been used as a high-performance system.

The prior-art antenna system, however, has been disadvantageous in that, since the principal antenna portions are rotated in the elevational and azimuthal directions along with the primary feed 4 and the communication equipment 5 with the rotation of the earth or the orbital drift of the satellite itself, it is not easy to install especially a low noise receiver or a high power amplifier in the communication equipment portion 5 in the vicinity of the primary feed portion. Also, the antenna structural size tends to be large, requiring an elevator or the like for the maintenance and operation to pro vide an access to the feed portion of the antenna. Likewise, the maintenance and addition of the communication equipment are subject to restrictions imposed by the size of the room for accommodating it. Furthermore, an unbalance in weight is caused by the elevational rotation,

in FIG. 3 showing a longitudinal section of an embodiment of the present invention, the portion 1 enclosed by dot dash lines denotes the principal portion; ll, the primary feed portion composed of waveguides; and ill. the communication equipment portion. The principal part i includes as the main constituent the antenna as shown in FIG. 1a, whose reference numerals are given to like constituents in this drawing. While the embodiment is handled as a transmitting antenna system in the following description, it is also applicable to a receiving antenna system. The electromagnetic wave generated by the communication equipment 5 is radiated from a throat point 0' of the electromagnetic horn l as a spherical wave. The wave is then reflected by a flat plate B and transformed into a spherical wave whose axis is bent by 90 relative to the initial axis Az. Thereafter, the spherical waves are further reflected by offset paraboloidal reflectors A and A which are in the relation of the image by inversion therebetween. They are thus focused as spherical waves, which are reflected by a plane reflector B. After they are focused at a focusing point 0, namely, the focus of the hyperboloid reflector 2, they are projected on the main reflector surface 2 in the form of a spherical wavefront. Then, they are reflected in the form of spherical waves with their center at the other focus of the reflector 2 and with a large spread angle. The reflected waves are converted into plane waves and reflected again by the paraboloid reflector 3 which has its focus in common with the reflector 2. The plane waves are projected with sharp directivity towards a target satellite which is located in the direction of the center axis of the reflector surface 3. Even in a communication with, for example, a gee-stationary satellite, the direction of the satellite changes with the rotation of the earth. The elevation and the azimuth angles of the antenna should therefore be varied so as to track the satellite. For this purpose, the antenna system is provided with a rotating mechanism with a rotary axis so that the elevation and the azimuth angles of the principal antenna may be varied. More specifically, the phantom line El horizontally drawn through the central part of the reflector plate B of the principal section I indicates the axis of rotation for changing the elevation angle 6. if the rotaty axis is rotated by the angle 0, the electromagnetic waves reflected by the plane reflector B are rotated by the angle 6 around the elevation drive axis El. The antenna assembled integrally with the plane reflector B is also rotated by the elevation angle 9 around the elevation drive axis El, so that the electromagnetic waves are radiated as a sharp radiation pattern in the direction in which the elevation angle of the antenna is inclined by the angle 6.

in the drawing, phanton line Az vertically drawn through the radiation point 0' and the central part of the plane reflector B of the primary feed portion II indicates the rotary axis for varying the azimuth angle (1). Upon rotation around the axis by the angle d), the vari ous elements of the primary feed portion ll and those of the principal part I are integrally rotated by the angle d1. Thus, the electromagnetic waves can be radiated with a sharp directivity in the direction in which the elevation angle of the antenna is inclined by the angle 9 and in which the azimuth angle is rotated by the angle d).

In general, when an axially symmetrical spherical wave radiated from a focus point is reflected by such a curved surface reflector obliquely with respect to the direction of incidence, the field distribution of the reflected electromagnetic waves changes asymmetrically. The present invention is aimed at the application to the beam waveguide primary feed portion of an antenna a new method by which the reflected electromagnetic waves thus having the radiation mode asymmetrically changed are reflected by another curved reflector to convert them into electromagnetic waves of the original axially symmetrical mode and then focus the reformed converted waves at the other focal point. This is defined as "mode matching." The operation will now be described with reference to the drawings.

in FlG. 40 showing the most fundamental type of arrangement for restoring the asymmetrically modemodified waves to the original mode in beam waveguides, the XY plane designates the section of a reference plane for symmetry; A and A, the sections of curved off-set paraboloidal reflectors; O, the radiation point of electromagnetic waves; O, the focusing point ofelectromagnetic waves; a and b, the ray-trace of the electromagnetic waves from the radiation point Q to both ends of the reflector section A; c and d, the raytrace of the electromagnetic wave between both ends of both the reflector sections A and A; and a and h, the ray-trace of the electromagnetic waves from both ends of the reflector section A to the focussing point 0. Since the curved off-set paraboloidal reflectors A and A are at positions of symmetry of the image-by inversion with respect to the reference plane XY, the raytrace a, h and u, b among those of the electromagnetic waves surrounded by broken lines are at the positions of symmetry of the image-by-inversion to each other.

FIG. 4b shows'an example where a plane reflector B is arranged in addition to the curved off-set paraboloidal surfaces A and A. Although the curved paraboloidal reflector A requires therefor the other curved paraboloidal reflector A of the image-by-inversion in correspondence thereto, the plane reflector B need have no corresponding reflecting plate.

With reference to FIGS. 50 and 5b, description will now be made of the reason why the curved off-set paraboloidal reflectors A and A should be arranged at the positions of the image by inversion. In FIG. 5a, the section of the curved off-set paraboloidal reflector A and the associated ray-trace of the electromagnetic waves are represented by the same symbols as in FIG. 4a. Broken line m indicates the path of the electromagnetic waves from the wave radiating point and in the direction in which the angle held by a and b is equally divided into two parts, while broken line n designates the path of the electromagnetic waves along which they proceed after being reflected by A. The off-set paraboloidal reflector A is of the paraboloid a revolution with its focal point at 0'. Therefore, the axially symmetrical electromagnetic waves emanating from the radiation point 0' are reflected by the reflector A, to direct them in the form of plane waves in the direction normal to the reference plane whose section is shown by XY. The plane of the reflector A is concave when viewed from the point Q. Therefore, m intersects with the section of the reflector A, at a point parting from its central portion towards its intersecting point with a, while it passes away from the middle line between c and d towards c. Accordingly, when the section of the reflected waves by the reflector A is observed at the reference plane whose section is shown at X Y, it is understood that the section of n is closer to the section of 0 than to the section of d. Thus, the mode of the reflected waves falls into the state in which the left is reduced while the right is enlarged. As a result, the field distribution of the TE wave of the fundamental dominant mode of the reflected waves becomes distorted as illustrated in the distribution of curves with arrows as given on the right side of an equation in FIG. b. Intrinsically, the TE wave must exhibit an electric field distribution shown at the last term on the left side of the equation in FIG. 5b. The generation of the aperture field distortion is equivalent to the fact that electromagnetic waves of undesired higher orders of including TE TE modes as shown in the first term, the second term, on the left side are generated and superposed. Accordingly, to use the electromagnetic waves having the field distribution as shown on the right side of the equation in FIG. 5b without any change is not desirable since it deteriorates the symmetry of the main beam pattern or picks up noise from the ground by spillover energy to thereby result in the axial shift of the tracking null pattern.

In contrast, if the curved off-set paraboloidal reflector A is arranged as illustrated in FIG. 4a, in the course of the electromagnetic waves in the sense of the image by inversion with respect to the curved off-set paraboloidal reflector A and thus the electromagnetic waves are reflected, then the radiation mode will revert again to the mode at the time of the radiation from the point 0' in the passage from the reflection by the reflector A to the focusing on the point 0. If only the dominant mode is radiated for the main beam from the point 0', electromagnetic waves of the dominant mode will be focused on the point 0. Accordingly, if the separate plane reflector B is employed so as to cause the subreflector of the dual reflector antenna to pass the focused waves through the point Q, only the electromagnetic waves of the dominant mode in beam waveguide will be radiated from the beam waveguide primary horn. Therefore, the main beam axis is coincident with the center axis thereof, and electromagnetic waves with low side lobes are radiated with a sharp directivity. Furthermore, when the electromagnetic waves of the tracking pattern are radiated from the point 0', the pattern is also focused on the point Q, and is projected from the dual reflector antenna. The tracking null pattern with the null axis directed to the center axis thereof can accordingly be made preciselyv Since the feed pattern does not undergo the generation of higher modes in beam waveguides, resulting in tracking pattern components, the tracking pattern is not influenced by the feed pattern at all. That is, a primary feed horn of excellent electrical performance can be produced for the first time by arranging the curved off-set paraboloidal reflectors A and A in the sense of the image by inversion to each other. On the contrary, the plane reflector does not require a flat plate conjugate thereto. This is because, as no change arises in the mode ol'electromagnetic waves by the reflection with the plane reflector, the reversion of the mode of reflected waves is unnecessary.

The application of the tracking pattern will now be stated. The tracking pattern is also formed by the highfrequency energy radiated from the electromagnetic horn 1 through the interior of the communication equipment 5, and is projected over the aperture of the dual reflector antenna through the same paths as in the case described above. However, for the purpose of selftracking the tracking pattern should preferably be like the TM and TE modes formed within a circular waveguide, in which the radiation in the central direction is zero with the peaks of the radiation lying in a slightly oblique direction. Since there is no radiation component in the direction of the front of the tracking pattern, the axis of the direction is termed the null axis. Although a change in mode also occurs as for the tracking pattern between the curved surface reflectors A and A of the primary feed portion II, it is not included at the focusing on the point 0. The tracking pattern can therefore be radiated with the null axis exactly directed to the front direction of the dual reflector antenna. Accordingly, the antenna system of the present invention has excellent electrical properties. Moreover, since the communication equipment 5 is located at the completely fixed ground, it is free from any trouble caused by gravity, and is very convenient for maintenance and operation. Also, the large room for accommodating the communication equipment facilitates the maintenance, remodeling, and addition of the antenna system.

Another embodiment of the present invention shown in FIG. 6 differs from the embodiment in FIG. 3 only in that the curved reflectors A and A of the primary feed portion II are composed of reflectors of an ellipsoid of revolution. The reflectors A and A are arranged with a point F as the center of rotational symmetry. Even if the change of mode arises in electromagnetic waves in the passage between the reflectors A and A, it will not be included therein at the focusing on the point Q. Accordingly, the beam axes and the null axis are also coincident with each other in case of the embodiment, noise due to spillover from beam waveguide are not picked up owing to the reduced side lobes. Thus, an antenna for satellite communication use is provided, which is capable of excellent selftracking.

The principle of the primary feed portion applied to the antenna system of the embodiment in FIG. 6 will now be described. As illustrated in HQ 7. electromagnetic waves reflected by the curved offset ellipsoidal reflector A" which is formed of a part of an ellipsoid of revolution having E and F as its foci are once focused on the point F, and are thereafter allowed to proceed as they are. They are reflected by curved reflector A which is formed of a part of another ellipsoid of revolution having F and G as its foci. Then, the reflected electromagnetic waves are focused on the point G. The mode of the electromagnetic waves radiated from the point E is changed after the reflection by the reflector A, in accordance with the principle explained with reference to FIG. b. After the passage through the point F, the direction of the change of the mode becomes the opposite. When the electromagnetic waves are focused on the point G after the reflection by the reflector A, the original mode is restored. In this case, the relation of the symmetry of the image by inversion is not held between such two curved reflectors A and A, and they are arranged at the positions of the point symmetry with each other with the center at the point F. With the curved offset ellipsoidal reflectors A and A in such arrangement, it is also possible to cancel the mode change generated in the electromagnetic waves by the reflection and to restore the original mode.

What is claimed is:

l. A microwave antenna system having a microwave source for radiating microwave energy in a predetermined direction with a preset disperse angle, a dual reflector antenna having a main reflector and a subreflector and microwave path for guiding said microwave energy to said antenna so that the microwave energy may be transmitted through said antenna most efficiently. wherein said microwave path comprises:

a first planar reflector for deflecting said microwave energy by a first curved reflector for deflecting the microwave energy by a predetermined angle;

a second curved reflector for deflecting the microwave reflected from said first curved reflector;

a second planar reflector for directing the reflected microwave supplied from said second curved reflector to the subreflector of said dual reflector antenna;

means for supporting said first planar reflector and first and second curved reflectors to permit said first and second curved reflectors to revolve about a first axis coincident with the axis of the microwave energy emanating from the microwave source and to permit said first planar reflector to rotate about said first axis;

means for supporting said dual reflector antenna and said second planar reflector to permit said antenna to revolve about a second axis lying at right angle to said first axis of the first-mentioned rotation and to permit said second planar reflector to rotate about said second axis; whereby the first-and second-mentioned rotations provides the control of the azimuthal and elevational direction of said dual reflector antenna without causing any transformation of the transmission mode of the microwave energy.

2. The antenna assembly of claim 1 wherein said first and second curved reflectors have concave parabaloidal contours serving as their reflecting surfaces. 

1. A microwave antenna system having a microwave source for radiating microwave energy in a predetermined direction with a preset disperse angle, a dual reflector antenna having a main reflector and a subreflector and microwave path for guiding said microwave energy to said antenna so that the microwave energy may be transmitted through said antenna most efficiently, wherein said microwave path comprises: a first planar reflector for deflecting said microwave energy by 90*; a first curved reflector for deflecting the microwave energy by a predetermined angle; a second curved reflector for deflecting the microwave reflected from said first curved reflector; a second planar reflector for directing the reflected microwave supplied from said second curved reflector to the subreflector of said dual reflector antenna; means for supporting said first planar reflector and first and second curved reflectors to permit said first and second curved reflectors to revolve about a first axis coincident with the axis of the microwave energy emanating from the microwave source and to permit said first planar reflector to rotate about said first axis; means for supporting said dual reflector antenna and said second planar reflector to permit said antenna to revolve about a second axis lying at right angle to said first axis of the first-mentioned rotation and to permit said second planar reflector to rotate about said second axis; whereby the first-and second-mentioned rotations provides the control of the azimuthal and elevational direction of said dual reflector antenna without causing any transformation of the transmission mode of the microwave energy.
 2. The antenna assembly of claim 1 wherein said first and second curved reflectors have concave parabaloidal contours serving as their reflecting surfaces. 